Piezoelectric actuator driver circuit

ABSTRACT

In a piezoelectric actuator driver circuit, a resistor provided to detect current is inserted in a current path for a piezoelectric actuator. A signal of a decreased voltage of the resistor is subjected to positive feedback to an amplifier circuit via a band-pass filter. An output signal of the amplifier circuit is subjected to negative feedback to the amplifier circuit via a band-elimination filter. The band-pass filter allows a signal of a fundamental resonant frequency of a piezoelectric device, which includes the piezoelectric actuator, to pass therethrough, and the band-elimination filter blocks the signal of the fundamental resonant frequency. Thus, a loop gain at a higher-order resonant frequency with respect to the fundamental resonant frequency becomes very low and a higher-order resonant mode can be effectively suppressed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a driver circuit for a piezoelectricactuator for vibrating a vibration body.

2. Description of the Related Art

Piezoelectric actuators typically include an electrode provided on amaterial having a piezoelectric effect, such as PZT ceramics, and arebasically voltage driven devices. In other words, mechanical deformationoccurs in response to a voltage applied to a piezoelectric actuator, andthe piezoelectric actuator typically must be resonantly driven. Resonantdriving is a driving scheme in which a piezoelectric actuator or astructure coupled therewith, hereinafter, referred to as “piezoelectricdevice,” causes a resonance phenomenon at a specific frequencydetermined by its mechanical shape and dimensions, thereby obtainingincreased deformation which cannot be obtained by normal voltageapplication.

In order to perform resonant drive, it is only necessary to apply analternating voltage at a resonant frequency of a piezoelectric device.For example, it is only necessary to connect an oscillator circuit,which generates an alternating voltage at the resonant frequency, to apiezoelectric device via a power amplifier.

However, individual differences between resonant frequencies ofpiezoelectric devices occur due to manufacturing variations ofpiezoelectric devices and inaccuracies in the mounting location ofpiezoelectric actuators on vibration bodies. Thus, it is difficult toresonantly drive a piezoelectric device merely by applying analternating signal having a fixed frequency determined previously forthe piezoelectric device. In addition, adjusting the frequency of analternating voltage applied to an individual piezoelectric device hasbeen considered. However, the resonant frequency of a piezoelectricdevice greatly changes with temperature changes, and thus, it isdifficult to stably resonantly drive a piezoelectric device even by withsuch measure.

Therefore, in the related art, a circuit has been proposed whichoperates to automatically determine the resonant frequency of apiezoelectric device and to generate an alternating signal at thefrequency and performs resonant drive with self-excited vibration. Asone example, an electrode and a terminal arranged to detect adeformation amount are provided in a piezoelectric actuator to define athree-electrode piezoelectric actuator, and a driver circuit is arrangedsuch that a drive signal is subjected to positive feedback to thepiezoelectric actuator by a signal from the terminal arranged to detecta deformation amount. In other words, this is a method in which thepiezoelectric actuator is controlled and driven such that itsdeformation amount is maximized.

However, a method of manufacturing such a three-electrode piezoelectricactuator is complicated and the cost is high. Further, in apiezoelectric actuator having a large amplitude of vibration, a largeamount of distortion occurs between a drive portion which deforms to alarge extent and a portion at which an electrode arranged to detect adeformation amount, which does not autonomously deform, is provided.Thus, the piezoelectric actuator is likely to be damaged.

When a two-electrode piezoelectric actuator is used which does notinclude the electrode arranged to detect a deformation amount, a circuitconfiguration can be used in which the piezoelectric actuator isincorporated into a resonance system of a driver circuit, such that thefrequency of an alternating voltage applied to the piezoelectricactuator is controlled to match the actual resonant frequency of thepiezoelectric actuator.

A known circuit which performs resonant drive with self-excitedvibration is disclosed in the Magazine “Fuel Cell”, written by KamiyaGaku, Kurihara Kiyoshi, and Hirata Atsuhiko, published by Fuel CellDevelopment Information Center, Apr. 30, 2009, VOL. 8, No. 4 2009, P148-151, FIG. 2. FIG. 1 shows a basic configuration of a driver circuitfor a piezoelectric actuator, which is shown the Magazine “Fuel Cell”,written by Kamiya Gaku, Kurihara Kiyoshi, and Hirata Atsuhiko, publishedby Fuel Cell Development Information Center, Apr. 30, 2009, VOL. 8, No.4 2009, P148-151, FIG. 2. A resistor R arranged to detect current isinserted in a current path for a piezoelectric actuator “a”. A voltagesignal proportional to a current flowing in the piezoelectric actuator“a” is obtained by the resistor R, and driving at a frequency at whichthe voltage-current phase difference of the piezoelectric actuator “a”is substantially 0° is achieved by an operational amplifier OP to whichpositive feedback of the voltage signal is provided.

The piezoelectric actuator driver circuit disclosed in the Magazine“Fuel Cell”, written by Kamiya Gaku, Kurihara Kiyoshi, and HirataAtsuhiko, published by Fuel Cell Development Information Center, Apr.30, 2009, VOL. 8, No. 4 2009, P148-151, FIG. 2, drives the piezoelectricactuator using its self vibration, and thus, the piezoelectric actuatorcan always be driven at a resonant frequency so as to correspond tovariations of the resonant frequency. However, a piezoelectric deviceincludes a plurality of higher-order resonant modes, in addition to afundamental resonant mode. These resonant modes are provided byvibration generated due to the shape and the size of the vibration bodyand the vibration of the piezoelectric actuator.

In the circuit shown in FIG. 1, positive feedback is provided at afrequency at which the impedance Z of the piezoelectric actuator “a”shows resistivity, i.e., at a frequency at which the reactance isapproximately 0, but positive feedback is provided even at ahigher-order resonant frequency other than the fundamental resonantfrequency. Thus, higher-order resonant oscillation is likely to occur.In the higher-order resonant oscillation, an oscillating portion cannotbe oscillated at a predetermined amplitude, and thus predeterminedfunctions of a piezoelectric device are not achieved. In addition,highly audible noise occurs due to harmonics.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide a driver circuit for a piezoelectric actuator,which is capable of performing stable self oscillation even whenvibrating a vibration body which is likely to cause higher-orderresonance.

A piezoelectric actuator driver circuit according to preferredembodiments of the present invention includes an amplifier circuitarranged to apply a drive voltage to a piezoelectric actuator whichvibrates an vibration body and to input to the piezoelectric actuator adetected signal generated in response to the drive voltage, and apositive feedback circuit arranged to provide positive feedback to theamplifier circuit and including a band-pass filter which allows afundamental resonant frequency of a piezoelectric device, which includesthe piezoelectric actuator provided to the vibration body, to passtherethrough.

The positive feedback circuit, i.e., a positive feedback loop, for theamplifier circuit is the same as a positive feedback circuit for thepiezoelectric actuator, which amplifies the voltage of the detectedsignal and provides positive feedback of the detected signal to thepiezoelectric actuator.

Further, a negative feedback circuit arranged to provide negativefeedback to the amplifier circuit, may preferably include aband-elimination filter which blocks a signal of the fundamentalresonant frequency of the piezoelectric device.

The band-elimination filter may be defined by, for example, aband-elimination filter which resonates at the fundamental resonantfrequency.

The vibration body may preferably include, for example, a plurality ofblades for a fan, and the band-elimination filter may preferably allow asignal of a higher-order resonant frequency caused by vibration of theplurality of blades to pass therethrough.

According to preferred embodiments of the present invention, even whenvibrating a vibration body which is likely to cause higher-orderresonance, the vibration body can be stably vibrated at the fundamentalresonant frequency.

This is because, by forming a harmonic suppression filter in thepositive feedback circuit for the amplifier circuit defined by aband-pass filter which allows the fundamental frequency of thepiezoelectric device to pass therethrough, a phase rotation angle aroundthe fundamental resonant frequency can be reduced, which results inallowing the piezoelectric actuator to be driven in a resistive state.In this case, the band-pass filter has a pass band such that the loopgain of the positive feedback is maintained high.

Further, by providing the band-elimination filter, which blocks thesignal of the fundamental resonant frequency of the piezoelectricdevice, in the negative feedback circuit for the amplifier circuit, theloop gain at the higher-order resonant frequency band is suppressed anda higher-order resonant mode is prevented.

Further, since the band-elimination filter blocks the fundamentalresonant frequency of the piezoelectric device, the difference (ratio)between the loop gains of the fundamental resonant frequency and thehigher-order resonant frequency can be large, and the higher-orderresonant mode can be more effectively prevented.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a driver circuit for a piezoelectric actuator,which is disclosed in the Magazine “Fuel Cell”, written by Kamiya Gaku,Kurihara Kiyoshi, and Hirata Atsuhiko, published by Fuel CellDevelopment Information Center, Apr. 30, 2009, VOL. 8, No. 4 2009,P148-151, FIG. 2.

FIG. 2 is a circuit diagram of a piezoelectric actuator driver circuitaccording to a first preferred embodiment of the present invention.

FIG. 3A is a perspective view of a piezoelectric fan which is an exampleof a piezoelectric device including a piezoelectric actuator.

FIG. 3B is a perspective view of a cooling unit including thepiezoelectric fan.

FIG. 4A is a frequency characteristic diagram of the impedance of apiezoelectric actuator in a state in which a piezoelectric device isprovided.

FIG. 4B is a frequency characteristic diagram of the phase of thepiezoelectric actuator.

FIG. 5 is a frequency characteristic diagram of a band-pass filter shownin FIG. 2.

FIG. 6A is a frequency characteristic diagram of a band-eliminationfilter shown in FIG. 2.

FIG. 6B is a frequency characteristic diagram of anotherband-elimination filter.

FIG. 7 is a circuit diagram of a piezoelectric actuator driver circuitaccording to a second preferred embodiment of the present invention.

FIG. 8 is a waveform diagram of each of: an applied voltage to a firstterminal of a piezoelectric actuator “a” shown in FIG. 7; an appliedvoltage to a second terminal thereof; and an applied voltage between theboth terminals of the piezoelectric actuator.

FIG. 9 is a circuit diagram of an inverting amplifier circuit arrangedto output a drive voltage to the piezoelectric actuator, a non-invertingamplifier circuit, and a feedback circuit arranged to detect a currentflowing in the piezoelectric actuator, in the piezoelectric actuatordriver circuit according to the second preferred embodiment of thepresent invention.

FIG. 10 is a circuit diagram of an amplifier circuit arranged to amplifyan output signal of the feedback circuit and to return the output signalto a balance driver circuit, a band-pass filter provided between aninput of the amplifier circuit and an output of the amplifier circuit, aband-elimination filter circuit in which a circuit is provided on anegative feedback side of the amplifier circuit, an automatic gaincontrol circuit, and a power supply circuit, in the piezoelectricactuator driver circuit according to the second preferred embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First PreferredEmbodiment

FIG. 2 is a circuit diagram of a piezoelectric actuator driver circuitaccording to a first preferred embodiment of the present invention. In acurrent path for a piezoelectric actuator “a”, a resistor R arranged todetect current is provided. A signal of a decreased voltage of theresistor R is subjected to positive feedback to an amplifier circuit AMPvia a band-pass filter BPF. An output signal of the amplifier circuitAMP is subjected to negative feedback to the amplifier circuit AMP via aband-elimination filter BEF. In this manner, a positive feedback loopPFL and a negative feedback loop NFL are provided.

The band-pass filter BPF allows a fundamental resonant frequency of apiezoelectric device, which includes the piezoelectric actuator “a”mounted on an vibration body, to pass therethrough, and blocks ahigher-order resonant frequency. In other words, the band-pass filterBPF functions as a harmonic suppression filter which blocks a signal ofa higher-order resonant frequency of the piezoelectric device. The gainof the amplifier circuit AMP is set such that a loop gain at thefundamental resonant frequency preferably exceeds 1, for example.

The piezoelectric actuator “a” is resistive, i.e., the reactancecomponent is approximately 0, at the resonant frequency of thepiezoelectric device. Thus, with a loop gain of approximately 1 or more,positive feedback is provided at the same phase, and thus, theBarkhausen vibration conditions at which the loop gain is approximately1 or more, and the phase angle is approximately 0° are satisfied and thepiezoelectric actuator oscillates at the fundamental resonant frequency.

On the other hand, the band-elimination filter BEF blocks a signalcomponent of the fundamental resonant frequency, and allows a signalcomponent of a higher-order resonant frequency to pass therethrough.Thus, the loop gain at the fundamental resonant frequency does notdecrease in the negative feedback loop NFL, and the loop gain at thehigher-order resonant frequency sufficiently decreases so as to be lessthan 1 by the negative feedback of the negative feedback loop NFL. Thus,the Barkhausen vibration conditions are not satisfied at thehigher-order resonant frequency, and vibration at the higher-orderresonant frequency is prevented.

FIG. 3A is a perspective view of a piezoelectric fan 1 which is anexample of a piezoelectric device including the piezoelectric actuator.FIG. 3B is a perspective view of a cooling unit including thepiezoelectric fan 1.

As shown in FIG. 3A, the piezoelectric fan 1 includes a vibration plate2 which is preferably a thin metal plate, such as a stainless plate, forexample. A plate-shaped substrate portion 2 a is provided on one edgeside of the vibration plate 2 extending in a length direction, andpiezoelectric elements 3 are attached to both surfaces of the substrateportion 2 a, thereby forming a bimorph piezoelectric actuator. Thevibration plate 2 is preferably bent at a bent portion 2 b at about 90°.A plurality of blades 2 d, for example, seven blades as shown in FIG.3A, are provided on the other edge side of the vibration plate 2extending in the length direction.

Each blade 2 d extends perpendicular or substantially perpendicular tothe principal surface direction of the piezoelectric elements 3. Anextension portion 2 c, in which the piezoelectric elements 3 are notattached, is provided on the end edge side of the substrate portion 2 aof the vibration plate 2, that is, at the edge of the substrate portion2 a opposite to the bent portion 2 b. The extension portion 2 c issupported by a support member 5 which is fixed to a fixing portion (notshown). The two piezoelectric elements 3 and the vibration plate 2 areelectrically connected to a piezoelectric actuator driver circuit 6.

As shown in FIG. 3B, the cooling unit includes the piezoelectric fan 1and a heat sink 10. The heat sink 10 includes a plurality of radiatingfins 11, for example, eight fins as shown in FIG. 3B, which are alignedat intervals. For example, the heat sink 10 is mounted on a top surfaceof a heat generating element, such as a CPU, which is mounted on acircuit substrate, so as to be thermally coupled thereto.

Each blade 2 d of the piezoelectric fan 1 is arranged between eachradiating fin 11 perpendicular or substantially perpendicular to abottom surface of the heat sink 10 in a non-contact manner. Thepiezoelectric actuator, including the substrate portion 2 a of thevibration plate 2 and the piezoelectric elements 3, is arranged so as tobe parallel or substantially parallel to and extend along an upper edgeof the heat sink 10.

As the vibration plate 2 is vibrated by the piezoelectric actuator, theblades 2 d vibrate parallel or substantially parallel to the sidesurfaces of the radiating fins 11, so as to fan the heat near theradiating fins 11 outward and away from the radiating fins 11. Thus, theheat sink 10 is efficiently cooled.

FIG. 4A is a frequency characteristic diagram of the impedance of thepiezoelectric actuator “a” provided in the piezoelectric device shown inFIG. 3B, and FIG. 4B is a frequency characteristic diagram of the phaseof the piezoelectric actuator “a”.

As shown in FIGS. 4A and 4B, a plurality of resonant points appear,however, the resonant point at the lowest frequency appears at about 95Hz. Thus, the piezoelectric actuator “a” resonates with a fundamentalwave at this frequency. In addition, a plurality of resonant pointsappear in the range of about 240 Hz to about 280 Hz. These points arethought to be caused by higher-order resonance caused by the vibrationof a plurality of the blades 2 d shown in FIGS. 3A and 3B. Note that thefrequency which actually provides the largest amplitude is slightlygreater than this range and is around the frequency at which the phaseangle is closest to 0°. In the drawing, the phase is not necessarilysufficiently close to 0°, and this is due to the frequency resolution atmeasurement being relatively low.

In the positive feedback loop of the circuit shown in FIG. 2, positivefeedback is preferably provided at a frequency at which the sum of thephase angle at the fundamental resonant frequency of the piezoelectricdevice and the phase angle at the central frequency of the passband ofthe band-pass filter BPF is approximately 0°, for example.

When a piezoelectric actuator, which resonates a plurality of blades, isused as described above, the resonant frequency is different for eachblade and thus complex resonant frequencies are likely to be generated.Therefore, preferred embodiments of the present invention are useful fora piezoelectric device having a piezoelectric actuator which resonates aplurality of blades.

FIG. 5 is a frequency characteristic diagram of the band-pass filter BPFshown in FIG. 2. In this example, the insertion loss at higher-orderresonant frequencies of about 240 Hz to about 280 Hz is about −3 dB whencompared to the insertion loss at the fundamental resonant frequency ofabout 95 Hz.

The band-pass filter BPF include a CR high-pass filter including aseries-connected capacitor and a shunt-connected resistor, and an RClow-pass filter including a series-connected resistor and ashunt-connected capacitor. Thus, the gradient is about −6 dB/oct in afrequency range sufficiently distant from a polar frequency. Byproviding such a BPF configuration, the phase rotation amount around thefundamental resonant frequency is preferably maintained at about 0°.Actually, the phase is close to 0° in the vicinity of 95 Hz in FIG. 5.

FIG. 6A is a frequency characteristic diagram of the band-eliminationfilter BEF shown in FIG. 2. FIG. 6B is a frequency characteristicdiagram of another band-elimination filter. In FIG. 6A, the attenuationamount at the fundamental resonant frequency of about 95 Hz is about −12dB, and outstanding attenuation occurs. The insertion loss athigher-order resonant frequencies of about 240 Hz to about 280 Hz isabout −0.5 dB or less, which is relatively low. In the example of FIG.6B, the attenuation amount at the fundamental resonant frequency ofabout 95 Hz is about −24 dB, and the insertion loss at higher-orderresonant frequencies of about 240 Hz to about 280 Hz is about 0 dB.

In this manner, by providing, in the negative feedback loop, theband-elimination filter BEF which allows nearly all of the signalcomponents of the higher-order resonant frequencies to pass therethroughand greatly attenuates the signal component of the fundamental resonantfrequency, the loop gain at the higher-order resonant frequencies can besufficiently decreased so as to be less than 1 while the loop gain atthe fundamental resonant frequency is maintained so as to be 1 or more.As a result, the vibration at the higher-order resonant frequencies canbe prevented while the stable vibration is maintained at the fundamentalresonant frequency.

Second Preferred Embodiment

FIG. 7 is a circuit diagram of a piezoelectric actuator driver circuitaccording to a second preferred embodiment of the present invention. Anamplifier circuit A21 amplifies a signal output from a feedback circuitA24, and supplies the amplified signal to a non-inverting amplifiercircuit A23. The non-inverting amplifier circuit A23 amplifies an outputvoltage of the amplifier circuit A21 with a predetermined gain, andapplies the amplified output voltage to a first terminal of thepiezoelectric actuator “a” via resistors R44 and R45. An invertingamplifier circuit A22 inverting-amplifies the output voltage of thenon-inverting amplifier circuit A23 with a gain of about 1, and appliesthe inverting-amplified output voltage to a second terminal of thepiezoelectric actuator “a”. The non-inverting amplifier circuit A23 andthe inverting amplifier circuit A22 define a balance driver circuit A25.

The feedback circuit A24 extracts a current, i.e., a detected signal,flowing in the piezoelectric actuator “a” in response to the applicationof the voltage to the piezoelectric actuator “a”, from both ends of theresistor R45, differential-amplifies the current, and supplies thedifferential-amplified current to a non-inverting input terminal of theamplifier circuit A21.

At both ends of the resistor R45, a voltage proportional to the currentflowing in the piezoelectric actuator “a” occurs. The feedback circuitA24 amplifies the voltage between both ends of the resistor R45 andoutputs an unbalanced signal. At this time, the output voltage of thefeedback circuit A24 is determined such that a positive feedback circuithaving a loop gain of more than 1 is preferably provided in the path ofA24→A21→A25. In other words, as the current flowing in the piezoelectricactuator “a” increases, the voltage applied to the piezoelectricactuator “a” increases.

A band-elimination filter BEF and an automatic gain control circuit areconnected to the negative feedback of the amplifier circuit A21. In theband-elimination filter BEF, the insertion loss of the signal componentof the fundamental resonant frequency of the piezoelectric device ishigh, and the insertion loss of the signal component of the higher-orderresonant frequency is low. In other words, the band-elimination filterBEF decreases the loop gain at the higher-order resonant frequency.

The automatic gain control circuit AGC detects an output voltage from anoutput terminal of the amplifier circuit A21 and controls the amount ofnegative feedback input to an inverting input terminal of the amplifiercircuit A21, so as to cause the voltage applied to the piezoelectricactuator “a” to be constant or substantially constant.

The amplitudes of the output voltages of the non-inverting amplifiercircuit A22 and the inverting amplifier circuit A23 are equal orsubstantially equal to a power supply voltage, and the phases of thesevoltages are opposite to each other. Thus, the piezoelectric actuator“a” is driven by the voltage which is approximately twice the powersupply voltage.

FIG. 8 is a waveform diagram of each of an applied voltage Va to thefirst terminal of the piezoelectric actuator “a” shown in FIG. 7, anapplied voltage Vb to the second terminal thereof, and an appliedvoltage Vab between both ends of the piezoelectric actuator “a”. Theamplifier circuits A22 and A23 preferably operate at a positive supplyof about +12 V and at a negative supply of about 0 V, for example, andthus, a voltage in the range of about 0 V to about +12 V is preferablyapplied to the first terminal A of the piezoelectric actuator “a” and avoltage in the range of about +12 V to about 0 V is preferably appliedto the second terminal B of the piezoelectric actuator “a”. Therefore,the applied voltage Vab between the both ends of the piezoelectricactuator “a” is (Va—Vb). In other words, about 24 Vp-p is applied as apeak-to-peak voltage.

FIGS. 9 and 10 are specific circuit diagrams of the piezoelectricactuator driver circuit shown in FIG. 7. The circuits in FIGS. 9 and 10are an integral circuit, but the integral circuit is divided into twoand shown for convenience of illustration. The circuit in FIG. 9 and thecircuit in FIG. 10 are connected to each other at terminals P1 and P2.

FIG. 9 shows the inverting amplifier circuit A22 which outputs a drivevoltage to the piezoelectric actuator “a”, the non-inverting amplifiercircuit A23, and the feedback circuit A24 which detects the currentflowing in the piezoelectric actuator “a”. The non-inverting amplifiercircuit A23 includes an operational amplifier OP6, resistors R12, R33,and R34, and a capacitor C10, and performs non-inverting amplificationwith a predetermined gain. The non-inverting amplifier circuit A23non-inverting-amplifies a signal input thereto from the terminal P2,with the predetermined gain, and supplies the non-inverting-amplifiedsignal to the first terminal of the piezoelectric actuator “a”.

The inverting amplifier circuit A22 includes an operational amplifierOP3, resistors R13 and R14, and a capacitor C11, and performs invertingamplification with a gain of about 1. In other words, the invertingamplifier circuit A22 inverting-amplifies the output signal of theamplifier circuit A23 at equal or substantially equal amplitude.

The inverting amplifier circuit A22 and the non-inverting amplifiercircuit A23 define the balance driver circuit A25. The feedback circuitA24 differential-amplifies a voltage between both ends of the resistorR30, and outputs the differential-amplified voltage to the terminal P1.

FIG. 10 shows the amplifier circuit A21 which amplifies an output signalof the feedback circuit A24 and returns the amplified output signal tothe non-inverting amplifier circuit A23 and the inverting amplifiercircuit A22, the band-pass filter BPF which is provided between an inputof the amplifier circuit A21 and an output of the feedback circuit A24,a band-elimination filter (BEF) circuit A27 in which a circuit isprovided on the negative feedback side of the amplifier circuit A21, anautomatic gain control (AGC) circuit A26, and a power supply circuit PS.

For example, the power supply circuit PS equally or substantiallyequally divides a power supply voltage DC12V using the resistors R31 andR32, and inputs a voltage as DC6V to a voltage follower circuit providedby an operational amplifier OP7, thereby generating a stable referencepotential VM (e.g., DC6V).

The band-pass filter BPF includes a low-pass filter including a resistorR5 and a capacitor C5, a high-pass filter including a capacitor C4 and aresistor R4, a low-pass filter including a resistor R3 and a capacitorC3, and a high-pass filter including a capacitor C2 and a resistor R2.Each cutoff frequency fc is obtained by 1/(2πRC).

The cutoff frequencies of the high-pass filters of two stages are lessthan the fundamental frequency of the piezoelectric device to which thepiezoelectric actuator “a” is mounted. In addition, the cutofffrequencies of the low-pass filters of two stages are greater than thefundamental frequency and less than the frequency of the secondharmonic. Therefore, the band-pass filter BPF allows the fundamentalfrequency to pass therethrough and suppresses harmonic components. Inother words, the band-pass filter BPF functions as a harmonicsuppression filter to suppress a signal of a higher-order resonantfrequency of the piezoelectric device. Thus, the frequency components ofharmonics are not subjected to positive feedback, the loop gain in thefrequency band of the harmonics is about 1 or less, and vibration doesnot occur in the harmonics. In other words, vibration occurs at thefundamental frequency of the piezoelectric device to which thepiezoelectric actuator “a” is mounted.

Note that, to suppress only the harmonic components, it is onlynecessary to provide predetermined stages of low-pass filters. However,an RC low-pass filter causes phase delays. Thus, preferably, the phaseshift amount is maintained at about 0 by providing the same stages of CRhigh-pass filters as that of the RC low-pass filters. The phase shiftamount is about −45° at the cutoff frequency of a single-stage RClow-pass filter, the phase shift amount is about −90° at a frequencysufficiently greater than the cutoff frequency, the phase shift amountis about +45° at the cutoff frequency of a single-stage CR high-passfilter, and the phase shift amount is about +90° at a frequencysufficiently less than the cutoff frequency, for example. Therefore, bytuning the cutoff frequency of each of the low-pass filters and thehigh-pass filters to the fundamental resonant frequency, positivefeedback can be provided at the fundamental resonant frequency in phase.

The amplifier circuit A21 is an element of a positive feedback circuit,i.e., a positive feedback loop, together with the amplifier circuitsA22, A23, and A24 and the piezoelectric actuator “a”, which are shown inFIG. 9. In addition, the amplifier circuit A21 is an element of anegative feedback circuit (negative feedback loop) together with the BEFcircuit A27 and the AGC circuit A26.

The BEF circuit A27 includes an operational amplifier OP2, resistors R6,R7, R8, R9, R10, and R11, and capacitors C6, C7, C8, and C9. Theresistors R9, R10, and R11 and the capacitors C7, C8, and C9 define aband-elimination filter (BEF) preferably using a Twin-T, for example. Bynon-inverting-amplifying a passing signal of the notch filter, theoperational amplifier OP2 causes an attenuation characteristic to besteeper, and decreases an output impedance around a frequency toresonate the piezoelectric actuator. Preferably, the BEF circuit A27 isset such that R9=R10=2*R11 and C8=C9=C7*½, and used in a state wheref0=1/(2π*R11*C7), for example. The amount of feedback to the midpoint ofthe Twin-T portion is set by R7 and R8. C6 and R6 are arranged to dividethe voltage of a signal from the AGC circuit A26, and to adjust a signalwhich returns to an amplifier circuit A11.

If settings are made such that R9=R10=2*R11 and C8=C9=C7*½, the cutofffrequency of the band-elimination filter (BEF) is obtained byf0=1/(2π*R11*C7), and is tuned to the vicinity of the frequency forresonating the piezoelectric device to which the piezoelectric actuator“a” is mounted.

An output signal of the BEF circuit A27 is subjected to negativefeedback by being input to an inverting input terminal of theoperational amplifier OP1 of the amplifier circuit A21. This negativefeedback signal is a signal which has passed through the BEF, and thus,only signals other than the fundamental resonant frequency are subjectedto negative feedback. As a result, the loop gain at the higher-orderresonant frequency is sufficiently suppressed and has a valuesignificantly less than 1, and vibration in harmonic frequencies isprevented. In other words, stable vibration occurs at the fundamentalfrequency.

The AGC circuit A26 includes a resistor R26, a capacitor C15, and afield-effect transistor T5. By the AGC circuit A26 being furtherconnected to the connection point between the BEF circuit A27 and theamplifier circuit A21, a path of the resistor R6 in the BEF circuitA27→the resistor R26 in the AGC circuit A26→the capacitor C15→thefield-effect transistor T5→the reference potential VM, is provided. Thispath is a variable attenuation circuit in which the resistance valuebetween the drain and the source of the field-effect transistor T5changes in response to an output signal from an operational amplifierOP4, and thus, the voltage dividing ratio changes in the path of theresistor R6 in the BEF A27→the resistor R26→the capacitor C15→thefield-effect transistor T5→the reference potential VM and theattenuation of a negative feedback signal divided between the resistorR26 and the resistor R6 and the capacitor C6 to the amplifier circuitA21 is effectively controlled. In other words, when the resistance valuebetween the drain and the source of the field-effect transistor T5changes, the voltage dividing ratio between the resistor R6 and theresistor R26 changes, and the amplitude of the negative feedback signalto the amplifier circuit A21 changes.

The operational amplifier OP4 in the AGC circuit A26 functions as avoltage comparator, and a reference voltage generation circuit in whicha power supply voltage Vcc is divided by resistors R18 and R19 and alow-pass filter arranged to stabilize a reference voltage, whichlow-pass filter includes a resistor R25 and a capacitor C14, areconnected to the non-inverting input terminal of the operationalamplifier OP4. A detector circuit arranged to rectify and detect anoutput signal from the amplifier circuit A21, which detector circuitincludes resistors R23 and R24, a diode D1, and a capacitor C13, isconnected to an inverting input terminal of the operational amplifierOP4.

In the operational amplifier OP4, when a detected voltage at theinverting input terminal from the amplifier circuit A21 is greater thanthe reference voltage at the non-inverting input terminal, the potentialof the output decreases. Thus, the resistance value between the drainand the source of the field-effect transistor T5 increases, and theamount of negative feedback to the amplifier circuit A21 increases.Therefore, the loop gain of the amplifier circuit A21 decreases, and thevibration output is suppressed.

On the other hand, in the operational amplifier OP4, when the detectedvoltage at the inverting input terminal from the amplifier circuit A21is greater than the reference voltage at the non-inverting inputterminal, the potential of the output increases. Thus, the resistancevalue between the drain and the source of the field-effect transistor T5decreases, and the amount of negative feedback to the amplifier circuitA21 decreases. Therefore, the loop gain of the amplifier circuit A21increases, and the vibration output increases. Note that, by providingthe resistor R22 and the capacitor C12, a time constant is provided, andthus, a slow operation is performed.

In this manner, the potential at the inverting input terminal of theoperational amplifier OP4 is always controlled so as to be equal orsubstantially equal to the reference potential at the non-invertinginput terminal thereof, thereby performing automatic gain control.

Note that, in the first and second preferred embodiments, the drivevoltage waveform of the piezoelectric actuator is preferably set so asto be a sine wave. However, by determining the loop gain under thecondition that the peak value of a sine wave exceeds the power supplyvoltage, the piezoelectric actuator may be driven with a trapezoidalwave or a rectangular wave, that is, a waveform in which the peakvoltage of the sine wave is clipped. When the occurrence of audiblenoise at audible frequencies, which is caused by a harmonic component ofthe trapezoidal wave or the rectangular wave, is a problem, it ispreferable to drive the piezoelectric actuator with a sine wave.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the invention. The scope of the invention, therefore, isto be determined solely by the following claims.

1. A piezoelectric actuator driver circuit comprising: an amplifiercircuit arranged to apply a drive voltage to a piezoelectric actuatorwhich vibrates a vibration body and to input to the piezoelectricactuator a detected signal generated in response to the drive voltage; apositive feedback circuit arranged to provide positive feedback to theamplifier circuit and including a band-pass filter arranged to allow afundamental resonant frequency of a piezoelectric device, which includesthe piezoelectric actuator attached to the vibration body, to passtherethrough; and a negative feedback circuit arranged to providenegative feedback to the amplifier circuit and including aband-elimination filter arranged to block a signal of the fundamentalresonant frequency of the piezoelectric device.
 2. The piezoelectricactuator driver circuit according to claim 1, wherein theband-elimination filter is a band-elimination filter which resonates atthe fundamental resonant frequency.
 3. The piezoelectric actuator drivercircuit according to claim 1, wherein the vibration body includes aplurality of fan blades; and the band-elimination filter allows a signalof a higher-order resonant frequency caused by vibration of theplurality of fan blades to pass therethrough.